Modern wireless communications systems for speech communications are commonly implemented using a speech coder operating at a fixed bit rate, a channel coder operating at a fixed bit rate, and a modulator operating at a fixed modulation format. These systems ordinarily rely on specific, modestly changing channel conditions, however, in a typical system, channel conditions are continuously changing and may experience dramatic variation. A problem with such systems is a failure to allocate optimal bit rates and modulation strategies for controlling the system elements based on current channel conditions.
For example, when an analog channel for speech communications has very little noise, existing systems do not take advantage of channel conditions. Alternatively, when the same channel degrades because of the presence of noise, existing systems do not compensate for such degrading channel conditions.
Typical network communication systems also experience changing channel conditions. For example, Internet packets (e.g., a set of bits of a predetermined size) may be transmitted via Internet Protocol. Systems using Internet Protocol typically deliver a packet without error and fail to deliver a packet with errors. Packets may also be delivered "late". Additionally, an intelligent router conveys packets via different communications paths based on system congestion. Speech communications systems using a network are designed to operate under modestly changing channel conditions. However, since channel conditions may change dramatically, a problem with such systems is an inability to adjust operating parameters when packets are delayed, lost, or out of sequence.
Another problem with existing systems is that speech quality degrades because of lower bit rate speech coding. Current systems fail to provide a graceful degradation to lower bit rate speech coding when packets are delayed, lost, or out of sequence.
Thus, what is needed are a system and method to control bit rates for a transmitter and a receiver based on changing network conditions. What is also needed are a system and method for allocating bits based on a packet error rate in a network. What is also needed are a system and method to provide graceful degradation for a low rate speech coder when a packet error rate increases for a network. What is also needed are a system and method to provide a synthetic speech output when voice packets are delayed, lost, or out of sequence between a transmitter and a receiver.